This invention relates to an optical glass of a SiO.sub.2 --B.sub.2 O.sub.3 --SrO--Li.sub.2 O system having optical constants of a refractive index (Nd) of 1.55-1.65 and an Abbe number (.nu.d) of 55-63, an excellent chemical durability and a relatively low softening point.
Known in the art are heavy crown system glasses having the above described optical constants. For example, the Glass Composition Data Book 1991 published by Japan Glass Products Industries Association describes many Sk (Schotcatalog name) types of glasses. These glasses, however, generally have a high yield point (At) exceeding 600.degree. C. and, for this reason, they require a high forming temperature. These glasses therefore are not suitable for mold pressing used for directly forming optical elements which do not require a grinding or polishing process after the press forming process. In mold presses, there arises the problem of shortening of the life of molds when the glass forming temperature is high and, for this reason, the yeild point of the glass used in these molds should be as low as possible.
For overcoming such problem, there has been proposed a glass of a system in which R'.sub.2 O (where R' represents an alkali metal) has been introduced. For example, Japanese Patent Application Laid-open No. Hei 4-292435 discloses a glass which contains a TeO.sub.2 ingredient as an essential element for preventing deterioration of chemical durability caused by merely introducing R'.sub.2 O in the conventional SiO.sub.2 --B.sub.2 O.sub.3 --BaO system glass. This glass, however, has not a sufficient acid-proof property. For another example, Japanese Patent Application Laid-open No. Sho 60-122747 discloses a glass of a SiO.sub.2 --B.sub.2 O.sub.3 --CaO--Li.sub.2 O system. This glass, however, is also insufficient in the acid-proof property and tends to have a discolored surface and therefore is not suitable for mold pressing.
It is, therefore, an object of the invention to provide an optical glass which has optical constants of a refractive index (Nd) of about 1.55-1.65 and an Abbe number (.nu.d) of about 55-63 and a yield point (At) which is suitable for mold pressing and yet has an excellent chemical durability.